Communications cables used for the transmission of information utilize two types of transmission media, copper or other metallic conductor media and optical fiber. There are also hybrid designs of communications cable that combine these two media.
Copper and optical fiber communications cables, while differing substantially in the mode in which they transmit information and even the form of energy they transfer, are nonetheless used in somewhat similar ways using similar installation methods. For instance, either cable may be used in terrestrial aerial applications or either may be buried directly in the ground.
Because copper cable is inherently metallic, unlike optical fiber cable which need not contain any metallic materials, that is all materials may be dielectric, special shielding is often necessary to protect that cable from the hazard of lightning strikes. Portions of suspended cable as well as surface exposed portions of buried cable may attract lightning. The problem with lightning strikes has been overcome partially by incorporating into the cable a metallic shield that encloses the circumference of the cable and extends longitudinally throughout the cable. The metallic shield is often disposed between an inner jacket and an outer jacket of the cable and is caused to be grounded at different points along the length of the cable. If lightning strikes a portion of the cable, the energy couples into the metallic shield and travels to ground, hopefully preventing damage to the transmission portion of the cable disposed within the metallic shield.
Optical fiber cables are often strengthened by incorporating metallic strength members into the sheath system thereof. Typically, a plurality of strengthening wires are disposed helically along the length of the cable before the cable receives a final outermost plastic jacket. Because these wires are metallic, they may attract lightning strikes to the cable. Lightning protection is provided for some lightguide cables in a method similar to that used to protect copper cable, that is, a metallic shield is disposed about an inner member of the cable sheath system during cabling. A final plastic jacket is disposed about the metallic shield.
It is desirable to be able to locate, for example, buried cables, for the purposes of maintenance and rearrangement and marking the path of the cable so as to avoid cutting the cable during future cable placement and digging operations. Buried cables, wires, pipes and other objects whose structures comprise continuous, longitudinally extending metal portions such as shields or strength members may be located by use of equipment referred to as cable locators. These so-called cable locators comprise two components, a signal transmitter and a signal receiver. The signal transmitter which is connected directly or inductively to a metallic portion of the buried structure and left in a stationary position is caused to transmit a so-called tracing tone into the structure. The tracing tone is an electric signal which causes the metallic portions of the buried object to radiate a characteristic electromagnetic field. An operator holds the receiver close to the ground and causes it to swing in a side-to-side motion above an area where the operator thinks the cable is located. The receiver is fitted with an electromagnetically inductive pick-up coil transducer which when excited by the electromagnetic field produced by the tracing tone produces a signal which can be interpreted to indicate the relative location of the buried object.
Recently, optical fiber cable sheath systems have been developed which provide the strength necessary for optical cable integrity without the use of metallic strength members. In U.S. Pat. No. 4,874,219 issued on Oct. 17, 1989 in the names of C. J. Arroyo, et al. there is disclosed an optical fiber cable system comprising an optical fiber core, a tube in which the core is disposed, a longitudinally extending water blocking tape, a plurality of helically wrapped longitudinally extending non-metallic strength members and an outer jacket. One of the benefits of this so called all-dielectric design is that, since it includes no metallic sheath system members, it does not attract lightning strikes nor will it transmit hazardous voltage if inadvertently crossed with power cables.
One of the drawbacks of the aforementioned all-dielectric design is that conventional buried cable locating techniques may not be used. If the buried cables do not contain any longitudinally extending metallic portions, then obviously the conventional means for locating buried cables described hereinbefore may not be used. What is needed and what seemingly is nowhere shown in the prior art are apparatus and methods for locating all-dielectric buried cables.